Pangolins are unique mammals, distinguished by their protective covering of overlapping scales made from keratin, similar to human fingernails. There are eight species of pangolins found across Asia and Africa, leading solitary and often nocturnal lives. In the early stages of the COVID-19 pandemic, these scaly creatures became a subject of significant public and scientific interest due to their potential connection to the SARS-CoV-2 virus. This article explores the scientific understanding of the “pangolin COVID” connection, examining why these animals were initially suspected and the broader context of zoonotic disease spread.
Why Pangolins Were Suspected
Pangolins became a focus in early SARS-CoV-2 origin investigations due to the discovery of coronaviruses within them showing genetic similarities to the pandemic virus. Researchers analyzed tissue samples from Malayan pangolins seized in southern China (2017-2018), identifying coronaviruses via RNA sequencing.
These pangolin-associated coronaviruses belonged to two sub-lineages related to SARS-CoV-2. One of these sub-lineages exhibited strong similarity in the receptor-binding domain (RBD) to SARS-CoV-2, which is the part of the virus that binds to host cells. This finding highlighted the RBD’s role as a determinant of host specificity. The presence of these related viruses in pangolins, particularly those from wildlife markets, led to the hypothesis that pangolins might have served as an intermediate host, facilitating the virus’s jump from bats to humans.
The Concept of Zoonotic Spillover
Zoonotic spillover describes the process where pathogens jump from animal populations to humans. Approximately 60-75% of emerging human infectious diseases have an animal origin. Spillover involves a complex interplay of factors that can hinder or facilitate pathogen transmission across species.
Factors influencing spillover include the intensity and frequency of interactions between different species, the genetic distance between host species, and specific characteristics of the pathogen itself. Environmental factors, such as habitat destruction and human encroachment, also play a role by forcing wildlife into closer proximity with human settlements and livestock, creating more opportunities for pathogens to cross species barriers.
Domestic animals, such as livestock, can sometimes act as intermediate hosts, becoming infected by wildlife and then transmitting the pathogen to humans. This highlights the interconnectedness of human, animal, and environmental health. Understanding these mechanisms is important for developing strategies to reduce spillover events and prevent future outbreaks.
Current Scientific Consensus on SARS-CoV-2 Origin
The scientific community continues to investigate the precise origin of SARS-CoV-2, with a consensus leaning towards a natural origin from bats. However, the exact evolutionary history and transmission pathway remain subjects of ongoing research. While pangolins were initially considered a strong candidate for an intermediate host, a direct and definitive link has not been conclusively established.
Further genomic analyses have shown that while pangolins are natural hosts for betacoronaviruses, the genetic similarity between pangolin coronaviruses and SARS-CoV-2 is 85.5% to 92.4%, while a bat coronavirus (RaTG13) shares approximately 96% whole-genome identity. This suggests that while pangolins may carry coronaviruses with a highly similar receptor-binding domain, the complete SARS-CoV-2 genome is more closely related to bat coronaviruses.
Some research indicates that SARS-CoV-2 may have resulted from a recombination event between bat and pangolin coronaviruses. Pangolins may also have been incidentally infected with a SARS-CoV-2-like virus from other animals in captivity or humans, rather than being the direct intermediate host. Research continues to explore these nuances, as multiple natural recombination events among viruses in bats and other wildlife species could have contributed to the emergence of SARS-CoV-2.